Silicone resin coating composition

ABSTRACT

Silicone resin coating compositions, which when applied to a solid substrate provide an abrasion resistant coating thereto, are disclosed herein. The coating compositions have a basic pH in the range of from 7.1-7.8 and are prepared by hydrolyzing an alkyltrialkoxysilane or aryltrialkoxysilane in an aqueous colloidal silica dispersion.

This is a continuation of application Ser. No. 964,910 filed Nov. 30,1978, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a protective coating composition. Moreparticularly, it relates to a silicone resin coating composition which,when applied to a substrate, forms a protective, abrasion-resistantcoating thereon.

Recently, the substitution of glass glazing with transparent materialswhich do not shatter or are more resistant to shattering than glass, hasbecome widespread. For example, transparent glazing made from syntheticorganic polymers is now utilized in public transportation vehicles, suchas trains, buses, taxis and airplanes. Lenses, such as for eye glassesand other optical instruments, as well as glazing for large buildings,also employ shatter-resistant transparent plastics. The lighter weightof these plastics in comparison to glass is a further advantage,especially in the transportation industry where the weight of thevehicle is a major factor in its fuel economy.

While transparent plastics provide the major advantage of being moreresistant to shattering and lighter than glass, a serious drawback liesin the ease with which these plastics mar and scratch, due to everydaycontact with abrasives, such as dust, cleaning equipment and/or ordinaryweathering. Continuous scratching and marring results in impariedvisibility and poor aesthetics, and oftentimes requires replacement ofthe glazing or lens or the like.

One of the most promising and widely used transparent plastics forglazing is polycarbonate, such as that known as Lexan®, sold by GeneralElectric Company. It is a tough material, having high impact strength,high heat deflection temperature, good dimensional stability, as well asbeing self-extinguishing, and is easily fabricated.

Attempts have been made to improve the abrasion resistance oftransparent plastics. For example, scratch-resistant coatings formedfrom mixtures of silica, such as colloidal silica or silica gel, andhydrolyzable silanes in a hydrolysis medium, such as alcohol and waterare known. U.S. Pat. Nos. 3,708,225, 3,986,997 and 3,976,497, forexample, describe such compositions.

While these afore-mentioned coating formulations have been foundacceptable, there still remains room for improvement. For example, thecoating compositions of this invention provide coatings having improvedresistance to moisture and humidity and ultraviolet light, in comparisonwith those coatings provided in accordance with U.S. Pat. No. 3,986,997.Moreover, it has been found herein that in direct contrast to theteachings of U.S. Pat. No. 3,986,997, the basic coating compositions ofthis invention having a pH in the range of from 7.1-7.8, do notimmediately gel and provide excellent coatings having good properties onsolid substrates.

Protective coatings for metals, bright or dull, are also needed. Forexample, bright metallized plastics, wherein small amounts of metal arevacuum sputtered or vacuum metallized over the plastic, now popular withthe auto industry because of their light weight, need protective layersto prevent scratching and marring of the brilliant surface. Metal wheelcovers (hub caps) also require protective coatings for theirpreservation and lasting beauty.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide a newprotective coating resin for solid substrates.

Another object of the invention is to provide a coating resin for solidsubstrates which, when applied to the substrate, will provide anabrasion-resistant surface thereto.

Still another object of the present invention is to provide a coatingcomposition, especially well suited for providing an abrasion resistantcoating surface to transparent substrates.

A further object of this invention is to provide a protective coatingresin which is readily applicable to a substrate and which, whenapplied, provides an improved coating having especially superiorresistance to moisture, humidity and ultraviolet light than prior artcoatings.

A still further object of the present invention is to provide a coatingresin composition also well suitable as a protective coating for metalsand metallized surfaces.

These and other objects are accomplished herein by a coating compositioncomprising a dispersion of colloidal silica in an aliphaticalcohol-water solution of the partial condensate of a silanol of theformula RSi(OH)₃, where R is selected from the group consisting of alkylhaving from 1 to 3 carbon atoms and aryl, at least 70 weight percent ofthe silanol being CH₃ --Si(OH)₃, said composition containing 10 to 50weight percent solids, said solids consisting essentially of 10 to 70weight percent colloidal silica and 30 to 90 weight percent of thepartial condensate, said composition having a pH from 7.1 to about 7.8.

DETAILED DESCRIPTION OF THE INVENTION

The coating compositions of this invention are prepared by hydrolyzing atrialkoxysilane or a mixture of trialkoxysilanes of the formulaRSi(OR)₃, wherein R is alkyl of from 1 to 3 carbons or aryl, such asphenyl, in an aqueous dispersion of colloidal silica.

In the practice of the present invention, suitable aqueous colloidalsilica dispersions generally have a particle size of from 5 to 150millimicrons in diameter. These silica dispersions are well known in theart and commercially available ones include, for example, those soldunder the trademarks of Ludox (duPont) and Nalcoag (NALCO Chemical Co.).Such colloidal silicas are available as both acidic and basic hydrosols.For the purpose of this invention, wherein the pH of the coatingcompositions is on the basic side, basic colloidal silica sols arepreferred. However, acidic colloidal silicas, wherein the pH is adjustedto a basic level, are also contemplated. In addition, it has been foundthat colloidal silicas having a low alkali content (e.g., Na₂ O) yield amore stable coating composition. Thus, colloidal silicas having analkali content of less than 0.35% (calculated as Na₂ O) have been foundto be preferable. Moreover, colloidal silicas having average particlesize of from 10 to 30 millimicrons are also preferred. A particularlypreferred aqueous colloidal silica dispersion for the purposes herein isknown as Ludox LS, sold by duPont Company.

In accordance with this invention, the aqueous colloidal silicadispersion is added to a solution of a small amount ofalkyltriacetoxysilane in alkyltrialkoxysilane or aryltrialkoxysilane.For the purposes herein, from about 0.07 parts by weight to about 0.1parts by weight, based on 100 parts by weight of the total composition,of the alkyltriacetoxysilane is used. The temperature of the reactionmixture is maintained at about 20° C. to about 40° C., preferably 20° C.to about 30° C., and most preferably below 25° C. It has been found thatin about six to eight hours sufficient trialkoxysilane has hydrolyzed soas to reduce the initial two phase liquid mixture to one liquid phase inwhich the now treated silica (i.e., treated by its admixture with thealkyltrialkoxysilane or aryltrialkoxysilane) is dispersed. In general,the hydrolysis reaction is allowed to continue for a total of about 24to 48 hours, depending upon the desired viscosity of the final product.The more time the hydrolysis reaction is permitted to continue, thehigher will be the viscosity of the product. After the hydrolysis hasbeen completed to the desired extent, the solids content is adjusted bythe addition of alcohol, preferably isobutanol, to the reaction mixture.Other suitable alcohols for the purposes herein include lower aliphaticalcohols such as methanol, ethanol, propanol, isopropanol, n-butylalcohol and t-butyl alcohol. Mixtures of such alcohols can be used, too.The solvent system should contain from about 20 to 75 weight percentalcohol to ensure solubility of the partial condensate (siloxanol).Optionally, additional water-miscible polar solvents, such as acetone,butyl cellosolve and the like in minor amounts, like no more than 20weight percent of the cosolvent system can also be used. The solidscontent of the coating compositions of this invention is generallypreferred to be in the range of from about 18 to 25%, most preferably,about 20%, by weight of the total composition. The pH of the resultantcoating composition is in the range of from about 7.1 to about 7.8,preferably higher than 7.2. If necessary, dilute base, such as ammoniumhydroxide or weak acid, such as acetic acid, can be added to thecomposition to adjust the final pH to this desired range. At these basicpH's, the compositions are translucent liquids which are stable at roomtemperature for at least several weeks. When stored at temperaturesbelow about 5° C. (40° F.) the period of stability is increased further.

Additives and other modifying agents, such as thickeners, pigments,dyes, and the like, may be added to the composition at this time. Aparticularly desirable additive to the coating compositions of thisinvention has been found to be a small amount of a polysiloxanepolyether copolymer. It has been found, and is the subject of anotherinvention disclosed in a copending U.S. application entitled "ImprovedSilicone Resin Coating Composition" Ser. No. 964,911, now U.S. Pat. No.4,277,287 to by Robert Bruce Frye, that these polysiloxane polyethercopolymers prevent the occurrence of undesirable flowmarks and dirtmarkswhich sometimes occur with the application of the coating compositiononto the substrate. A particularly useful polysiloxane polyethercopolymer for the purposes herein is known as SF-1066 and is availablefrom the General Electric Company. The preparation, further descriptionand structural formulae for these polysiloxane polyether copolymers arefound in U.S. Pat. No. 3,629,165, incorporated by reference herein.

The alkyltriacetoxysilane is used to buffer the basicity of the initialtwo liquid phase reaction mixture and thereby also temper the hydrolysisrate. While the use of alkyltriacetoxysilane is preferred herein,glacial acetic acid may be used in its place, as well as other acidssuch as organic acids like propionic, butyric, citric, benzoic, formic,oxalic and the like. Alkyltriacetoxysilanes wherein the alkyl groupcontains from 1 to 6 carbon atoms can be used, alkyl groups having from1 to 3 carbon atoms being preferred. Methyltriacetoxysilane is mostpreferred for the purposes herein.

The silanetriols, RSi(OH)₃, hereinbefore mentioned, are formed in situas a result of the admixture of the corresponding trialkoxysilanes withthe aqueous medium, i.e., the aqueous dispersion of colloidal silica.Exemplary trialkoxysilanes are those containing methoxy, ethoxy,isopropoxy and n-butoxy substituents which, upon hydrolysis, generatethe silanetriol and further liberate the corresponding alcohol. In thisway, at least, a portion of the alcohol content present in the finalcoating composition is provided. Of course, if a mixture oftrialkoxysilanes is employed as provided for hereinabove, a mixture ofdifferent silanetriols, as well as different alcohols, is generated.Upon the generation of the silanetriol or mixtures of silanetriols inthe basic aqueous medium, condensation of the hydroxyl substituents toform ##STR1## bonding occurs. This condensation takes place over aperiod of time and is not an exhaustive condensation but rather thesiloxane retains an appreciable quantity of siliconbonded hydroxylgroups which render the polymer soluble in the alcohol-water cosolvent.This soluble partial condensate can be characterized as a siloxanolpolymer having at least one siliconbonded hydroxyl group per every three##STR2## units.

The non-volatile solids portion of the coating composition herein is amixture of colloidal silica and the partial condensate (or siloxanol) ofa silanol. The major portion or all of the partial condensate orsiloxanol is obtained from the condensation of CH₃ Si(OH)₃ and,depending upon the input of ingredients to the hydrolysis reaction,minor portions of partial condensate can be obtained, for example, fromthe condensation of CH₃ Si(OH)₃ with C₂ H₅ Si(OH)₃ or C₃ H₇ Si(OH)₃ ;CH₃ Si(OH)₃ with C₆ H₅ Si(OH)₃, or even mixtures of the foregoing. Foroptimum results in the cured coating it is preferred to use allmethyltrimethoxysilane (thus generating all monomethylsilanetriol) inpreparing the coating compositions herein. In the preferred coatingcompositions herein the partial condensate is present in an amount offrom about 55 to 75 weight percent of the total solids in a cosolvent ofalcohol and water, the alcohol comprising from about 50% to 95% byweight of the cosolvent.

The coating compositions of this invention will cure on a substrate attemperatures of, for example, 120° C. without the aid of an added curingcatalyst. However, in order to employ more desirable milder curingconditions, buffered latent condensation catalysts can be added.Included in this class of catalysts are alkali metal salts of carboxylicacids, such as sodium acetate, potassium formate and the like. Aminecarboxylates, such as dimethylamine acetate, ethanolamine acetate,dimethylaniline formate and the like, quaternary ammonium carboxylatessuch as tetramethylammonium acetate, benzyltrimethylammonium acetate,metal carboxylates, like tin octoate and amines such as triethylamine,triethanolamine, pyridine and the like are also contemplated curingcatalysts herein. Alkali hydroxides, like sodium hydroxide and ammoniumhydroxide can also be used as curing catalysts herein. Moreover, typicalcommercially available colloidal silica, especially those having a basicpH, contain free alkali metal base and alkali metal carboxylatecatalysts will be generated in situ during the hydrolysis reactionherein.

The amount of curing catalyst can be varied within a wide range,depending upon the desired curing conditions. However, in general,catalyst in the amounts of from about 0.05 to about 0.5 weight percent,preferably about 0.1 weight percent, of the composition can be used.Compositions containing catalysts in these amounts can be cured on asolid substrate in a relatively short time at temperatures in the rangeof from about 75°-150° C. to provide a transparent abrasion resistantsurface coating.

The coating compositions of the present invention can be applied to avariety of solid substrates by conventional methods, such as flowing,spraying or dipping, to form a continuous surface film. Substrates whichare especially contemplated herein are transparent, as well asnon-transparent, plastics and metals. More particularly, these plasticsare synthetic organic polymeric substrates such as acrylic polymers,like poly(methylmethacrylate), polyesters, such as poly(ethyleneterephthalate), poly(butylene terephthalate), etc., polyamides,polyimides, acrylonitrile-styrene copolymers,styrene-acrylonitrile-butadiene copolymers, polyvinyl chloride,butyrates, polyethylene and the like. As noed above, the coatingcompositions of this invention are especially useful as coatings forpolycarbonates, such as those polycarbonates known as Lexan®, sold byGeneral Electric Company. The metal substrates on which the presentprotective coatings are utilized include bright and dull metals likealuminum and bright metallized surfaces like sputtered chromium alloy.Other solid substrates contemplated herein include wood, paintedsurfaces, leather, glass, ceramics and textiles.

By choice of the proper formulation, application conditions andpretreatment, including the use of primers, of the substrate, thecoatings can be adhered to substantially all solid substrates. A hardcoating having all of the aforementioned characteristics and advantagesis obtained by the removal of the solvent and volatile materials. Thecoating composition will air-dry to a tack-free condition, but heatingin the range of 75° C. to 200° C. is necessary to obtain condensation ofresidual silanols in the partial condensate. This final cure results inthe formation of silsequioxane (RSiO_(3/2)). In the finished curedcoating the ratio of RSiO_(3/2) units to SiO₂ will range from about 0.43to about 9.0, preferably 1 to 3. A cured coating having a ratio ofRSiO_(3/2) to SiO₂, where R is methyl, of 2 is most preferred. Thecoating thickness can be varied by means of the particular applicationtechnique, but coatings of about 0.5 to 20 microns, preferably 2-10micron thickness are generally utilized.

In order that those skilled in the art may better understand how topractice the present invention, the following examples are given by wayof illustration and not by way of limitation.

EXAMPLE 1

22.1 parts by weight of Ludox LS, silica sol (aqueous dispersion ofcolloidal silica, average particle size of 12 millimicrons, pH of 8.2,sold by duPont) is added to a solution of 0.1 parts by weight ofmethytriacetoxysilane in 26.8 parts by weight of methyltrimethoxysilane.The temperature of the reaction mixture is maintained at 20° C. to 30°C., preferably below 25° C. The hydrolysis is allowed to continue for 24hours. The solids content of the resultant reaction mixture is 40.5% andis diluted to about 20% solids with the addition of isobutanol thereto.The pH of the product is about 7.2.

This composition is flow-coated onto a transparent Laxan®(poly(bisphenol-A carbonate)) panel primed with a thermosetting acrylicemulsion. After air drying for 30 minutes the panel is cured for onehour at 120° C. After 500 Taber Abraser cycles (500 g load, CS-10Fwheels) (ANSI-Z26.1-1977 section 5.17) the change in percent haze (Δ%H)is 1.8. The sample passes the cross-hatched adhesion test (DIN-53-151)after 7 days in 65° C. water. It also passes the cross-hatched adhesiontest (DIN-53-151) after 1000 hours under R-S Lamp exposure on a rotatingplatform 10" from the faces of an array of 6 lamps arranged at 120° fromone another. The platform rotates at about 3 RPM.

EXAMPLE 2

300 grams of a 30% aqueous colloidal silica dispersion having an averageparticle size of 20 millimicrons and pH of 9.2 (prepared by dilution ofNalcoag 1050, a product of NALCO Chemical Company) is added to asolution of 0.9 grams glacial acetic acid and 366 grams ofmethyltrimethoxysilane. The temperature of the reaction mixture ismaintained below 30° C. The hydrolysis is allowed to continue for 3days. The solids content of the resultant reaction mixture is 40.5% andis diluted with 419 grams of isopropanol to a final dilution of 25%solids. The pH of the final composition is about 7.5.

This composition (containing 3% of a 3% solution of tetramethylammoniumacetate) is flow-coated on a transparent Lexan® panel primed with anethanol-isobutanol solution containing 1.5 weight percent ofgamma-aminopropyltriethoxysilane and 1.5 wt. % of a preformed reactionproduct of gamma-aminopropyltriethoxysilane and maleic anhydride asdisclosed in copending U.S. application Ser. No. 901,543, filed on May1, 1978, now U.S. Pat. No. 4,246,038 by Holub and Vaughn. After airdrying for 30 minutes the panel is cured for 1 hour at 120° C. After 500Taber Abraser cycles the change in percent haze (Δ%H) is 1.15.

EXAMPLE 3

17.9 grams of Ludox LS is added to a solution of 0.06 grams of glacialacetic acid in 27.2 grams of methyltrimethoxysilane. The temperature ofthe reaction mixture is maintained below 30° C. The hydrolysis isallowed to continue for 6 hours. The solids content is 41.6% and isdiluted with 30 grams of isobutanol to a final solids content of 25%.The pH is about 7.2.

The composition (containing 3% of a 3% solution of tetramethylammoniumacetate) is flow-coated on a transparent Lexan® panel primed as inExample 2. After curing, the (Δ%TH) of the panel is 1.6.

Obviously, other modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that changes may be made in the particular embodimentsdescribed above which are within the full intended scope of theinvention as defined in the appended claims.

I claim:
 1. An aqueous coating composition comprising a dispersion ofcolloidal silica, having a particle size of from about 5 to about 150millimicrons in diameter, in a lower aliphatic alcohol-water solution ofthe partial condensate of a silanol of the formula RSi(OH)₃, wherein Ris selected from the group consisting of alkyl having from 1 to 3 carbonatoms and phenyl, at least 70 weight percent of the silanol being CH₃Si(OH)₃, said composition containing 10 to 50 weight percent solidsconsisting essentially of 10 to 70 weight percent of the partialcondensate, said composition having a pH of 7.1 to about 7.8.
 2. Acoating composition as defined in claim 1 wherein the aliphatic alcoholis a mixture of methanol and isobutanol.
 3. A coating composition asdefined in claim 1, wherein said partial condensate is of CH₃ Si(OH)₃.4. A coating composition as defined in claim 1 containing from about0.05 to about 0.5 weight percent of a buffered latent silanolcondensation catalyst.
 5. A coating composition as defined in claim 4wherein said catalyst is sodium acetate.
 6. A coating composition asdefined in claim 4 wherein said catalyst is tetramethylammonium acetate.7. A coating composition as defined in claim 1 wherein the pH is about7.2 to 7.8.
 8. A coating composition as defined in claim 1 wherein thecomposition contains from about 18 to 25 weight percent solidsconsisting essentially of 25-45 weight percent colloidal silica and55-75 weight percent of the partial condensate.
 9. A coating compositionas defined in claim 8 wherein the partial condensate is of CH₃ Si(OH)₃.10. A coating composition as defined in claim 1 wherein the compositioncontains about 20% solids, the partial condensate is of CH₃ Si(OH)₃ andthe aliphatic alcohol is a mixture of methanol and isobutanol.
 11. Anaqueous coating composition prepared by admixing an aqueous colloidalsilica dispersion having a particle size of from about 5 to about 150millimicrons in diameter with a solution of an alkyltriacetoxy silane inan alkyltrialkoxysilane, said alkyl groups having from 1 to 3 carbonatoms, maintaining the temperature of the mixture at from about 20° C.to about 30° C. for a sufficient time to from a partial condensate of asilanol of the alkyltriacetoxy silane and the alkyltrialkoxy silane, atleast 70 weight percent of the silanol being CH₃ Si(OH)₃, and to reducethe reaction mixture to one liquid phase, maintaining the pH of thecomposition in the range of from about 7.1 to 7.8, and adjusting thesolids content of the reaction mixture to 10 to 50 weight percent solidsconsisting essentially of 10 to 70 weight percent colloidal silica and30 to 90 weight percent of the partial condensate by the addition of alower aliphatic alcohol thereto.
 12. An aqueous coating composition asdefined in claim 11 wherein said alkyltrialkoxysilane ismethyltrimethoxysilane, said alkyl(triacetoxy)silane ismethyl(triacetoxy)silane and said aliphatic alcohol is isobutanol. 13.An aqueous coating composition as defined in claim 12 wherein saidaqueous colloidal silica dispersion has a basic pH, an average particlesize of about 12 microns and an alkali content of about 0.10%.